The once-great show now all too often plays like a pale imitation of its old self.

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Perhaps this season’s silliest – and most unintentionally revealing – episode of “Glee” came last month when singing student Sam pretended to be his slicker, smarter (and nonexistent) twin brother.

The contrivance, unlike “Glee” gimmicks of years past, came off as forced and annoying. The bit also can be seen as a sign of the sad decline of a once-great show that now all too often plays like a pale imitation of its old self.

“Glee,” which heads for its Season 4 finale Thursday, hasn’t quite jumped the shark. But it shot itself in the foot at times this season – and not just during the ill-conceived school gunfire episode.

Somewhere along the way, the once-thrilling high-wire mix of song, comedy, teenage (and adult) angst and occasional heartfelt moments, lost its footing. All the Gleeks among us can hope is that “Glee” somehow bounces back Thursday and leaves for summer vacation on, well, a high note.

The creators faced a major challenge after last season, in which some of the main characters, played by actors old enough to be out of medical school, finally graduated McKinley High. Juggling newcomers with old characters proved a formidable task, especially for a show whose success rests in maintaining a delicate balance amid a breathless pace.

But this transitional season has groaned with growing pains – and missteps. Some criticized last month’s school gunfire plotline, believing it came too soon after the Newtown, Conn. tragedy. The timing proved less troubling than the handling of the sequence, which came across less as drama than as emotional manipulation. Meanwhile, the subsequent jokey school blackout episode transpired as if the frightening shooting incident never happened.

“Glee” found ways in the past to present serious issues, sans heavy hands – taking on faith in the great “Grilled Cheesus” episode, and bullying, homophobia, sexual identity and teen suicide in the compelling Kurt-Dave Karofsky storyline. When “Glee” failed, it failed big – and memorably (wheelchair-using Artie “walking” on Christmas morning in Season 2).

The show has gone from daring to more than occasionally tiresome, rife with repetition, particularly with the revolving love matches. It’s hard to care at this point whether Will and Emma will ever be happy together. It’s also difficult to become invested in some of the new students, some of whom are too much like the old students – namely Puck’s brother, Jake, and Quinn clone Kitty. Unique and Marley show great potential, but their development hasn’t soared in sync with their impressive voices. (Speaking of songs, it’s been mixed bag this season – did we really need a second Britney Spears-themed installment? Still, the latest episode’s Stevie Wonder playlist returned the show to higher ground, at least musically).

Bringing back the old faces helps, but not when they’re returning to the refuge of McKinley. The device works better – if not always smoothly – when placing the familiar characters outside the comfort of high school, as we saw with Rachel, Kurt and Santana’s move to New York.

The graduates, like the show, need to grow up – or risk growing old fast. If high school is about a search for identity, then “Glee” needs to carve out a new one. Sam, meanwhile, should just stick to being Sam.

While we’ve come close at times to changing the channel, Fox isn’t giving up on “Glee,” renewing it for two more seasons. We watch out of hope the show will entertain us the way it once did – and because we still care about some of the characters born with “Glee.”

In Thursday’s season finale, Rachel goes for a second callback for a Broadway revival of “Funny Girl,” the role she was born to play. Even if “Glee” rained on its own parade much of this season, it hasn’t been enough of a downpour to fill the shark tank – at least not yet. Check out a promo below for the finale, which also includes the Glee club’s latest bid to win Regionals: